The present invention relates in general to blast heating, and in particular to a method of operating a blast heating device or regenerator in which heat contained in flue gas after the discharge of the latter from the regenerator is transferred to combustion air and fuel dust by means of an indirect heat exchanger operating with a liquid heat-transfer medium, so that the combustion air and fuel dust be preheated by the flue gas.
In a blast heater or regenerator pertaining to a high furnace, the furnace blast to be heated and flue gases of higher temperature generated in the combustion spaces of the regenerator are alternately flown through the latter. The flue gases result during the combustion of gaseous and/or liquid fuels. For economical operation of a blast furnace a blast temperature of about 1250.degree. C. is normally required. As a consequence, the temperature of flue gas employed for heating the regenerator must have a correspondingly higher value, and at the inlet in the regenerator amounts to about 1450.degree. C. Such high flue gas temperature can be achieved for example by combusting a mixture of waste gas from a blast furnace with waste coke oven gas or natural gas, but also by combusting preheated waste gas from the blast furnace together with preheated combustion air. Alternatively, it is also possible to use a combination of the two aforementioned methods, namely a small preheating of the combustion media and simultaneously admixing coke oven waste gas or natural gas.
Since furnace blasts fed in the regenerator alternately with the flue gas are, from operational and technological reasons, compressed to pressures of about 4 to 5 bar, it has at the entry into the regenerator already a relatively high temperature, for example of 150.degree. C. This temperature of course conditions the final temperature of the flue gas at its discharge from the regenerator, to be also relatively high. For example, the final temperature of the flue gas at its exit from the regenerator amounts to about 200.degree. C. at the beginning of the heating phase and about 300.degree. C. at the end of this phase. In periods of low cost of energy, the flue gas having this temperature is normally discharged through a chimney, inasmuch as the costs of investment for the energy recovery would be higher than the value of the saved energy.
In periods when the cost of energy substantially increases, this economic consideration is completely changed, and both new power plants and existing, old plants are being equipped with heat-regenerating devices.
In the case of new power plants this problem is normally solved in such a manner that flue gases exhausted from the regenerator are immediately guided through a heat exchanger with the combustion air and fuel gas, in which tangible heat is indirectly transferred to the combustion gas and to the fuel gas fed into combustion spaces of the regenerator. It is also known how to bring flue gases before entry in the aforementioned heat exchanger to a higher temperature by means of a series-connected combustion chamber in which gases having low calorific value are burned, so that the preheating temperatures of the combustion air and fuel gas are further increased. This known system, however, has the disadvantage that both the combustion air and fuel gas, as well as flue gas, must be fed through the same heat exchanger. Consequently, gas conduits must in this case be designed with a large diameter, and the heat exchanger must be located in close proximity to the regenerator, and all remaining conduits must be accurately adjusted to this system. For this reason, this prior-art system is suitable primarily for installation in newly built power plants only. If this system is applied to extant old plants, it is necessary to make extensive and costly reconstructions in the type of conduits, and in addition local space conditions frequently make the reconstruction impossible.
A heat-exchanging system has been developed especially for use in reconstruction of existing, old power plants. In such this system, the heat exchanger for the flue gas and the heat exchangers for the combustion air, as well as for the fuel gas, are separated from each other and coupled together by a suitable liquid heat-transferring medium which is recirculated. In such known system, heat of flue gas upon its discharge from the regenerator is first transferred to the liquid heat-transferring medium, such as for example alkyldiphenyl, which in turn transfers its heat energy to the combustion air and to the fuel gas in the aforementioned separate heat exchangers. The system of this kind is suitable particularly for the additional installation into an extant power plant, because the individual heat exchangers can be connected separately to the existing pipe conduits or, alternatively, can bypass the latter. In this manner, expensive redesign and changes in the pipe conduits are generally avoided.
Flue gas exhausted from the regenerator at a certain temperature from the regenerator can naturally preheat the combustion media to this temperature, for example to 150.degree. C. If, however, it is desired to use waste gas from a blast oven as the fuel gas, then these preheating temperatures no longer suffice for bringing the combustion spaces in the regenerator to the desired high combustion temperature of about 1450.degree. C., for example. In using the above-described separate heat-exchanging system, it is therefore necessary to admit to the waste gas from the blast furnace a calorically rich fuel such as for example waste coke oven gas, or natural gas, or fuel oil, to obtain the high combustion temperature. However, this measure is disadvantageous, inasmuch as waste gas from blast furnaces is the byproduct of the production of pig iron and is abundantly available at low costs, whereas fuel of high calorific value, to be admixed, must be bought at high price or withdrawn from another application in the range or outside the complex of metallurgical plants.